Professor Gaffney leads a research team focused on femtosecond resolution measurements of chemical dynamics in complex condensed phase systems. This research takes advantage of recent advances in ultrafast x-ray lasers, like the LCLS at SLAC National Accelerator Laboratory, to directly observe chemical reactions on the natural time and length scales of the chemical bond – femtoseconds and Ångströms. This research focuses on the discovery of design principles for controlling the non-equilibrium dynamics of electronic excited states and using these principles to spark new approaches to light-driven catalysis in chemical synthesis.

This research builds on Professor Gaffney’s extensive experience with ultrafast optical laser science and technology. This work began with time- and angle- resolved two photon photoemission studies of electron solvation and localization at interfaces as a graduate student working with Professor Charles Harris at the University of California at Berkeley and extended to multidimensional vibrational spectroscopy studies of hydrogen bonding and ion solvation dynamics in solution during postdoctoral studies with Professor Michael Fayer at Stanford and as an Assistant Professor. The transition to ultrafast x-ray science began in 2004 at SLAC, where he helped establish the first chemical dynamics research program at SLAC.

Academic Appointments

  • Associate Professor, Photon Science Directorate
  • Associate Professor (By courtesy), Chemistry

Administrative Appointments

  • Department chair, Photon Science Department (2020 - Present)
  • Chemical Sciences Division Director, Energy Sciences Directorate, SLAC National Accelerator Laboratory (2019 - Present)
  • Deputy Associate Laboratory Director, Energy Sciences Directorate, SLAC National Accelerator Laboratory (2019 - Present)
  • Associate Laboratory Director, Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory (2014 - 2019)
  • Principle Investigator, PULSE Institute (2007 - Present)

Professional Education

  • PhD, University of California at Berkeley, Chemistry (2001)

Current Research and Scholarly Interests

The research team Professor Gaffney leads focuses on time resolved studies of chemical reactions. Recent advances in ultrafast x-ray lasers, like the LCLS at SLAC National Accelerator Laboratory, enable chemical reactions to be observed on the natural time and length scales of the chemical bond – femtoseconds and Ångströms. The knowledge gained from x-ray and optical laser studies will be used to spark new approaches to photo-catalysis and chemical synthesis.

2021-22 Courses

Stanford Advisees

All Publications

  • Capturing Atom-Specific Electronic Structural Dynamics of Transition-Metal Complexes with Ultrafast Soft X-Ray Spectroscopy. Annual review of physical chemistry Jay, R. M., Kunnus, K., Wernet, P., Gaffney, K. J. 1800


    The atomic specificity of X-ray spectroscopies provides a distinct perspective on molecular electronic structure. For 3d metal coordination and organometallic complexes, the combination of metal- and ligand-specific X-ray spectroscopies directly interrogates metal-ligand covalency-the hybridization of metal and ligand electronic states. Resonant inelastic X-ray scattering (RIXS), the X-ray analog of resonance Raman scattering, provides access to all classes of valence excited states in transition-metal complexes, making it a particularly powerful means of characterizing the valence electronic structure of 3d metal complexes. Recent advances in X-ray free-electron laser sources have enabled RIXS to be extended to the ultrafast time domain. We review RIXS studies of two archetypical photochemical processes: charge-transfer excitation in ferricyanide and ligand photodissociation in iron pentacarbonyl. These studies demonstrate femtosecond-resolution RIXS can directly characterize the time-evolving electronic structure, including the evolution of the metal-ligand covalency. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see for revised estimates.

    View details for DOI 10.1146/annurev-physchem-082820-020236

    View details for PubMedID 34985923

  • Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering CHEMICAL SCIENCE Gaffney, K. J. 2021; 12 (23): 8010-8025


    Light-driven chemical transformations provide a compelling approach to understanding chemical reactivity with the potential to use this understanding to advance solar energy and catalysis applications. Capturing the non-equilibrium trajectories of electronic excited states with precision, particularly for transition metal complexes, would provide a foundation for advancing both of these objectives. Of particular importance for 3d metal compounds is characterizing the population dynamics of charge-transfer (CT) and metal-centered (MC) electronic excited states and understanding how the inner coordination sphere structural dynamics mediate the interaction between these states. Recent advances in ultrafast X-ray laser science has enabled the electronic excited state dynamics in 3d metal complexes to be followed with unprecedented detail. This review will focus on simultaneous X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) studies of iron coordination and organometallic complexes. These simultaneous XES-XSS studies have provided detailed insight into the mechanism of light-induced spin crossover in iron coordination compounds, the interaction of CT and MC excited states in iron carbene photosensitizers, and the mechanism of Fe-S bond dissociation in cytochrome c.

    View details for DOI 10.1039/d1sc01864g

    View details for Web of Science ID 000656555500001

    View details for PubMedID 34194691

    View details for PubMedCentralID PMC8208315

  • Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nature chemistry Biasin, E., Fox, Z. W., Andersen, A., Ledbetter, K., Kjar, K. S., Alonso-Mori, R., Carlstad, J. M., Chollet, M., Gaynor, J. D., Glownia, J. M., Hong, K., Kroll, T., Lee, J. H., Liekhus-Schmaltz, C., Reinhard, M., Sokaras, D., Zhang, Y., Doumy, G., March, A. M., Southworth, S. H., Mukamel, S., Gaffney, K. J., Schoenlein, R. W., Govind, N., Cordones, A. A., Khalil, M. 2021


    It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (FeiiRuiii) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of angstroms, 120-200cm-1 frequency and ~100fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.

    View details for DOI 10.1038/s41557-020-00629-3

    View details for PubMedID 33589787

  • Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers CHEMICAL SCIENCE Kunnus, K., Li, L., Titus, C., Lee, S., Reinhard, M. E., Koroidov, S., Kjaer, K. S., Hong, K., Ledbetter, K., Doriese, W. B., O'Neil, G. C., Swetz, D. S., Ullom, J., Li, D., Irwin, K., Nordlund, D., Cordones, A., Gaffney, K. J. 2020; 11 (17): 4360–73

    View details for DOI 10.1039/c9sc06272f

    View details for Web of Science ID 000532365500005

  • Atomistic characterization of the active-site solvation dynamics of a model photocatalyst NATURE COMMUNICATIONS van Driel, T. B., Kjaer, K. S., Hartsock, R. W., Dohn, A. O., Harlang, T., Chollet, M., Christensen, M., Gawelda, W., Henriksen, N. E., Kim, J. G., Haldrup, K., Kim, K. H., Ihee, H., Kim, J., Lemke, H., Sun, Z., Sundstrom, V., Zhang, W., Zhu, D., Moller, K. B., Nielsen, M. M., Gaffney, K. J. 2016; 7


    The interactions between the reactive excited state of molecular photocatalysts and surrounding solvent dictate reaction mechanisms and pathways, but are not readily accessible to conventional optical spectroscopic techniques. Here we report an investigation of the structural and solvation dynamics following excitation of a model photocatalytic molecular system [Ir2(dimen)4](2+), where dimen is para-diisocyanomenthane. The time-dependent structural changes in this model photocatalyst, as well as the changes in the solvation shell structure, have been measured with ultrafast diffuse X-ray scattering and simulated with Born-Oppenheimer Molecular Dynamics. Both methods provide direct access to the solute-solvent pair distribution function, enabling the solvation dynamics around the catalytically active iridium sites to be robustly characterized. Our results provide evidence for the coordination of the iridium atoms by the acetonitrile solvent and demonstrate the viability of using diffuse X-ray scattering at free-electron laser sources for studying the dynamics of photocatalysis.

    View details for DOI 10.1038/ncomms13678

    View details for Web of Science ID 000388643800001

    View details for PubMedID 27892472

    View details for PubMedCentralID PMC5133712

  • Large Angular Jump Mechanism Observed for Hydrogen Bond Exchange in Aqueous Perchlorate Solution SCIENCE Ji, M., Odelius, M., Gaffney, K. J. 2010; 328 (5981): 1003-1005


    The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate despite extensive experimental and theoretical studies. We have applied polarization-selective multidimensional vibrational spectroscopy to investigate the H-bond exchange mechanism in aqueous NaClO4 solution. The results show that a water molecule shifts its donated H-bonds between water and perchlorate acceptors by means of large, prompt angular rotation. Using a jump-exchange kinetic model, we extracted an average jump angle of 49 +/- 4 degrees, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaClO4 solution.

    View details for DOI 10.1126/science.1187707

    View details for Web of Science ID 000277877100034

    View details for PubMedID 20489019

  • Quantifying the Steric Effect on Metal-Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering. Inorganic chemistry Kunnus, K., Guo, M., Biasin, E., Larsen, C. B., Titus, C. J., Lee, S. J., Nordlund, D., Cordones, A. A., Uhlig, J., Gaffney, K. J. 1800


    Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L3 edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)2]2+ and [Fe(btbip)2]2+, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe-carbene bond length due to steric effects modify the metal-ligand bonding, including sigma/pi donation and pi back-donation. We find that sigma donation is significantly stronger in [Fe(bmip)2]2+, whereas the pi back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)2]2+ lead to approximately 1 eV destabilization of the quintet metal-centered 5T2g excited state compared to [Fe(btbip)2]2+, providing an explanation for the absence of a photoinduced 5T2g population and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)2]2+. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.

    View details for DOI 10.1021/acs.inorgchem.1c03124

    View details for PubMedID 35029978

  • Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering. Journal of the American Chemical Society Larsen, C. B., Braun, J. D., Lozada, I. B., Kunnus, K., Biasin, E., Kolodziej, C., Burda, C., Cordones, A. A., Gaffney, K. J., Herbert, D. E. 2021


    The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

    View details for DOI 10.1021/jacs.1c06429

    View details for PubMedID 34851636

  • Direct observation of ultrafast hydrogen bond strengthening in liquid water. Nature Yang, J., Dettori, R., Nunes, J. P., List, N. H., Biasin, E., Centurion, M., Chen, Z., Cordones, A. A., Deponte, D. P., Heinz, T. F., Kozina, M. E., Ledbetter, K., Lin, M., Lindenberg, A. M., Mo, M., Nilsson, A., Shen, X., Wolf, T. J., Donadio, D., Gaffney, K. J., Martinez, T. J., Wang, X. 2021; 596 (7873): 531-535


    Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network1, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments2-7. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04A on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch.

    View details for DOI 10.1038/s41586-021-03793-9

    View details for PubMedID 34433948

  • Short-lived metal-centered excited state initiates iron-methionine photodissociation in ferrous cytochrome c. Nature communications Reinhard, M. E., Mara, M. W., Kroll, T., Lim, H., Hadt, R. G., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Driel, T. B., Hartsock, R. W., Kjaer, K. S., Weninger, C., Biasin, E., Gee, L. B., Hodgson, K. O., Hedman, B., Bergmann, U., Solomon, E. I., Gaffney, K. J. 2021; 12 (1): 1086


    The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c. This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1s3p (Kbeta) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state.

    View details for DOI 10.1038/s41467-021-21423-w

    View details for PubMedID 33597529

  • Photodissociation of aqueous I3- observed with liquid-phase ultrafast mega-electronvolt electron diffraction Structural Dynamics Ledbetter, K., et al 2020; 21: 10


    Developing femtosecond resolution methods for directly observing structural dynamics is critical to understanding complex photochemical reaction mechanisms in solution. We have used two recent developments, ultrafast mega-electron-volt electron sources and vacuum compatible sub-micron thick liquid sheet jets, to enable liquid-phase ultrafast electron diffraction (LUED). We have demonstrated the viability of LUED by investigating the photodissociation of tri-iodide initiated with a 400 nm laser pulse. This has enabled the average speed of the bond expansion to be measured during the first 750 fs of dissociation and the geminate recombination to be directly captured on the picosecond time scale.

    View details for DOI 10.1063/4.0000051

    View details for PubMedCentralID PMC7771998

  • Origin of core-to-core x-ray emission spectroscopy sensitivity to structural dynamics. Structural dynamics (Melville, N.Y.) Vacher, M., Kunnus, K., Delcey, M. G., Gaffney, K. J., Lundberg, M. 2020; 7 (4): 044102


    Recently, coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of Kalpha x-ray emission spectroscopy (XES). Understanding the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics is important for further development of femtosecond time-resolved XES methods and, we believe, generally necessary for interpretation of XES signals from highly non-equilibrium structures that are ubiquitous in photophysics and photochemistry. Here, we use multiconfigurational wavefunction calculations combined with atomic theory to analyze the emission process in detail. The sensitivity of core-to-core transitions to structural dynamics is due to a shift of the minimum energy metal-ligand bond distance between 1s and 2p core-hole states. A key effect is the additional contraction of the non-bonding 3s and 3p orbitals in 1s core-hole states, which decreases electron-electron repulsion and increases overlap in the metal-ligand bonds. The effect is believed to be general and especially pronounced for systems with strong bonds. The important role of 3s and 3p orbitals is consistent with the analysis of radial charge and spin densities and can be connected to the negative chemical shift observed for many transition metal complexes. The XES sensitivity to structural dynamics can be optimized by tuning the emission energy spectrometer, with oscillations up to ±4% of the maximum intensity for the current system. The theoretical predictions can be used to design experiments that separate electronic and nuclear degrees of freedom in ultrafast excited state dynamics.

    View details for DOI 10.1063/4.0000022

    View details for PubMedID 32665965

  • XFELs: cutting edge X-ray light for chemical and material sciences PHYSICAL CHEMISTRY CHEMICAL PHYSICS Asakura, K., Gaffney, K. J., Milne, C., Yabashi, M. 2020; 22 (5): 2612–14

    View details for DOI 10.1039/c9cp90304f

    View details for Web of Science ID 000517561500001

    View details for PubMedID 31912832

  • Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering. Nature communications Kunnus, K., Vacher, M., Harlang, T. C., Kjar, K. S., Haldrup, K., Biasin, E., van Driel, T. B., Papai, M., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Kallman, E., Delcey, M., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Laursen, M. G., Hansen, F. B., Vester, P., Christensen, M., Sandberg, L., Nemeth, Z., Szemes, D. S., Bajnoczi, E., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Lemke, H. T., Canton, S. E., Moller, K. B., Nielsen, M. M., Vanko, G., Warnmark, K., Sundstrom, V., Persson, P., Lundberg, M., Uhlig, J., Gaffney, K. J. 2020; 11 (1): 634


    The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip=2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kalpha and Kbeta X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kalpha XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1s and 2p core-ionized states surfaces.

    View details for DOI 10.1038/s41467-020-14468-w

    View details for PubMedID 32005815

  • Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy Journal of Chemical Physics Ledbetter, K., Reinhard, M. E., Kunnus, K., Gallo, A., Britz, A., Biasin, E., Glownia, J. M., Nelson, S., Van Driel, T. B., Weninger, C., Zederkof, D. B., Haldrup, K., Cordones, A. A., Gaffney, K. J., Sokaras, D., Alonso-Mori, R. 2020; 152

    View details for DOI 10.1063/1.5139441

  • Simulations of valence excited states in coordination complexes reached through hard X-ray scattering. Physical chemistry chemical physics : PCCP Källman, E. n., Guo, M. n., Delcey, M. G., Meyer, D. A., Gaffney, K. J., Lindh, R. n., Lundberg, M. n. 2020; 22 (16): 8325–35


    Hard X-ray spectroscopy selectively probes metal sites in complex environments. Resonant inelastic X-ray scattering (RIXS) makes it is possible to directly study metal-ligand interactions through local valence excitations. Here multiconfigurational wavefunction simulations are used to model valence K pre-edge RIXS for three metal-hexacyanide complexes by coupling the electric dipole-forbidden excitations with dipole-allowed valence-to-core emission. Comparisons between experimental and simulated spectra makes it possible to evaluate the simulation accuracy and establish a best-modeling practice. The calculations give correct descriptions of all LMCT excitations in the spectra, although energies and intensities are sensitive to the description of dynamical electron correlation. The consistent treatment of all complexes shows that simulations can rationalize spectral features. The dispersion in the manganese(iii) spectrum comes from unresolved multiple resonances rather than fluorescence, and the splitting is mainly caused by differences in spatial orientation between holes and electrons. The simulations predict spectral features that cannot be resolved in current experimental data sets and the potential for observing d-d excitations is also explored. The latter can be of relevance for non-centrosymmetric systems with more intense K pre-edges. These ab initio simulations can be used to both design and interpret high-resolution X-ray scattering experiments.

    View details for DOI 10.1039/d0cp01003k

    View details for PubMedID 32236271

  • Femtosecond electronic structure response to high intensity XFEL pulses probed by iron X-ray emission spectroscopy. Scientific reports Alonso-Mori, R. n., Sokaras, D. n., Cammarata, M. n., Ding, Y. n., Feng, Y. n., Fritz, D. n., Gaffney, K. J., Hastings, J. n., Kao, C. C., Lemke, H. T., Maxwell, T. n., Robert, A. n., Schropp, A. n., Seiboth, F. n., Sikorski, M. n., Song, S. n., Weng, T. C., Zhang, W. n., Glenzer, S. n., Bergmann, U. n., Zhu, D. n. 2020; 10 (1): 16837


    We report the time-resolved femtosecond evolution of the K-shell X-ray emission spectra of iron during high intensity illumination of X-rays in a micron-sized focused hard X-ray free electron laser (XFEL) beam. Detailed pulse length dependent measurements revealed that rapid spectral energy shift and broadening started within the first 10 fs of the X-ray illumination at intensity levels between 1017 and 1018 W cm-2. We attribute these spectral changes to the rapid evolution of high-density photoelectron mediated secondary collisional ionization processes upon the absorption of the incident XFEL radiation. These fast electronic processes, occurring at timescales well within the typical XFEL pulse durations (i.e., tens of fs), set the boundary conditions of the pulse intensity and sample parameters where the widely-accepted 'probe-before-destroy' measurement strategy can be adopted for electronic-structure related XFEL experiments.

    View details for DOI 10.1038/s41598-020-74003-1

    View details for PubMedID 33033373

  • Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy. Chemical science Kjar, K. S., Van Driel, T. B., Harlang, T. C., Kunnus, K., Biasin, E., Ledbetter, K., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Li, L., Laursen, M. G., Hansen, F. B., Vester, P., Christensen, M., Haldrup, K., Nielsen, M. M., Dohn, A. O., Papai, M. I., Moller, K. B., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Jarenmark, M., Uhlig, J., Sundstom, V., Warnmark, K., Persson, P., Nemeth, Z., Szemes, D. S., Bajnoczi, E., Vanko, G., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Canton, S. E., Lemke, H. T., Gaffney, K. J. 2019; 10 (22): 5749–60


    Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such experiments, it becomes possible to identify the dominant trajectory followed during the excited state cascade and to determine the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2'-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered molecular systems involving 3d transition metals.

    View details for DOI 10.1039/c8sc04023k

    View details for PubMedID 31293761

  • Ultrafast X-Ray Scattering Measurements of Coherent Structural Dynamics on the Ground-State Potential Energy Surface of a Diplatinum Molecule PHYSICAL REVIEW LETTERS Haldrup, K., Levi, G., Biasin, E., Vester, P., Laursen, M., Beyer, F., Kjaer, K., van Driel, T., Harlang, T., Dohn, A. O., Hartsock, R. J., Nelson, S., Glownia, J. M., Lemke, H. T., Christensen, M., Gaffney, K. J., Henriksen, N. E., Moller, K. B., Nielsen, M. M. 2019; 122 (6): 063001


    We report x-ray free electron laser experiments addressing ground-state structural dynamics of the diplatinum anion Pt_{2}POP_{4} following photoexcitation. The structural dynamics are tracked with <100  fs time resolution by x-ray scattering, utilizing the anisotropic component to suppress contributions from the bulk solvent. The x-ray data exhibit a strong oscillatory component with period 0.28 ps and decay time 2.2 ps, and structural analysis of the difference signal directly shows this as arising from ground-state dynamics along the PtPt coordinate. These results are compared with multiscale Born-Oppenheimer molecular dynamics simulations and demonstrate how off-resonance excitation can be used to prepare a vibrationally cold excited-state population complemented by a structure-dependent depletion of the ground-state population which subsequently evolves in time, allowing direct tracking of ground-state structural dynamics.

    View details for DOI 10.1103/PhysRevLett.122.063001

    View details for Web of Science ID 000458824200006

    View details for PubMedID 30822093

  • Initial metal-metal bond breakage detected by fs X-ray scattering in the photolysis of Ru-3(CO)(12) in cyclohexane at 400 nm PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES Kong, Q. Y., Laursen, M. G., Haldrup, K., Kjaer, K. S., Khakhulin, D., Biasin, E., van Driel, T. B., Wulff, M., Kabanova, V., Vuilleumier, R., Bratos, S., Nielsen, M. M., Gaffney, K. J., Weng, T. C., Koch, M. J. 2019; 18 (2): 319–27


    Using femtosecond resolution X-ray solution scattering at a free electron laser we were able to directly observe metal-metal bond cleavage upon photolysis at 400 nm of Ru3(CO)12, a prototype for the photochemistry of transition metal carbonyls. This leads to the known single intermediate Ru3(CO)11(μ-CO)*, with a bridging ligand (μCO) and where the asterisk indicates an open Ru3-ring. This loses a CO ligand on a picosecond time scale yielding a newly observed triple bridge intermediate, Ru3(CO)8(μ-CO)3*. This loses another CO ligand to form the previously observed Ru3(CO)10, which returns to Ru3(CO)12via the known single-bridge Ru3(CO)10(μ-CO). These results indicate that contrary to long standing hypotheses, metal-metal bond breakage is the only chemical reaction immediately following the photolysis of Ru3(CO)12 at 400 nm. Combined with previous picosecond resolution X-ray scattering data and time resolved infrared spectroscopy these results yield a new mechanism for the photolysis of Ru3(CO)12.

    View details for DOI 10.1039/c8pp00420j

    View details for Web of Science ID 000458569100033

    View details for PubMedID 30628601

  • Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy. Angewandte Chemie (International ed. in English) Tatsuno, H. n., Kjaer, K. S., Kunnus, K. n., Harlang, T. C., Timm, C. n., Guo, M. n., Chàbera, P. n., Fredin, L. A., Hartsock, R. W., Reinhard, M. E., Koroidov, S. n., Li, L. n., Cordones, A. A., Gordivska, O. n., Prakash, O. n., Liu, Y. n., Laursen, M. G., Biasin, E. n., Hansen, F. B., Vester, P. n., Christensen, M. n., Haldrup, K. n., Németh, Z. n., Sárosiné Szemes, D. n., Bajnóczi, É. n., Vankó, G. n., Van Driel, T. B., Alonso-Mori, R. n., Glownia, J. M., Nelson, S. n., Sikorski, M. n., Lemke, H. T., Sokaras, D. n., Canton, S. E., Dohn, A. O., Møller, K. B., Nielsen, M. M., Gaffney, K. J., Wärnmark, K. n., Sundström, V. n., Persson, P. n., Uhlig, J. n. 2019


    Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an FeII NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3 MLCT state, from the initially excited 1 MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3 MC state, in competition with vibrational relaxation and cooling to the relaxed 3 MLCT state. The relaxed 3 MLCT state then decays much more slowly (7.6 ps) to the 3 MC state. The 3 MC state is rapidly (2.2 ps) deactivated to the ground state. The 5 MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3 MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.

    View details for DOI 10.1002/anie.201908065

    View details for PubMedID 31602726

  • Soft X-ray spectroscopy with transition-edge sensors at Stanford Synchrotron Radiation Lightsource beamline 10-1 Review of Scientific Instruments Lee, S., Titus, C. J., Alonso-Mori, R., Baker, M. L., Bennett, D. A., Cho, H., Doriese, W. B., Fowler, J. W., Gaffney, K. J., Gallo, A., Gard, J. D., Hilton, G. C., Jang, H., Jo, Y., Kenney, C. J., Knight, J., Kroll, T., Lee, J., Li, D., Lu, D., Marks, R., Minitti, M. P., Morgan, K. M., Ogasawara, H., O'Neil, G. C., et al 2019; 90

    View details for DOI 10.1063/1.5119155

  • Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Noninnocent Ligands by Ni L-Edge and S K-Edge X-ray Absorption INORGANIC CHEMISTRY Koroidov, S., Hong, K., Kjaer, K. S., Li, L., Kunnus, K., Reinhard, M., Hartsock, R. W., Amit, D., Eisenberg, R., Das Pemmaraju, C., Gaffney, K. J., Cordones, A. A. 2018; 57 (21): 13167–75
  • Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering JOURNAL OF PHYSICAL CHEMISTRY LETTERS Jay, R. M., Norell, J., Eckert, S., Hantschmann, M., Beye, M., Kennedy, B., Quevedo, W., Schlotter, W. F., Dakovski, G. L., Minitti, M. P., Hoffmann, M. C., Mitra, A., Moeller, S. P., Nordlund, D., Zhang, W., Liang, H. W., Kunnus, K., Kubicek, K., Techert, S. A., Lundberg, M., Wernet, P., Gaffney, K., Odelius, M., Foehlisch, A. 2018; 9 (12): 3538–43


    Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time-resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. π-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences σ-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.

    View details for PubMedID 29888918

  • Fingerprints of electronic, spin and structural dynamics from resonant inelastic soft X-ray scattering in transient photo-chemical species PHYSICAL CHEMISTRY CHEMICAL PHYSICS Norell, J., Jay, R. M., Hantschmann, M., Eckert, S., Guo, M., Gaffney, K. J., Wernet, P., Lundberg, M., Foehlisch, A., Odelius, M. 2018; 20 (10): 7243–53


    We describe how inversion symmetry separation of electronic state manifolds in resonant inelastic soft X-ray scattering (RIXS) can be applied to probe excited-state dynamics with compelling selectivity. In a case study of Fe L3-edge RIXS in the ferricyanide complex Fe(CN)63-, we demonstrate with multi-configurational restricted active space spectrum simulations how the information content of RIXS spectral fingerprints can be used to unambiguously separate species of different electronic configurations, spin multiplicities, and structures, with possible involvement in the decay dynamics of photo-excited ligand-to-metal charge-transfer. Specifically, we propose that this could be applied to confirm or reject the presence of a hitherto elusive transient Quartet species. Thus, RIXS offers a particular possibility to settle a recent controversy regarding the decay pathway, and we expect the technique to be similarly applicable in other model systems of photo-induced dynamics.

    View details for DOI 10.1039/c7cp08326b

    View details for Web of Science ID 000429286100052

    View details for PubMedID 29484313

    View details for PubMedCentralID PMC5885270

  • Anisotropy enhanced X-ray scattering from solvated transition metal complexes JOURNAL OF SYNCHROTRON RADIATION Biasin, E., van Driel, T. B., Levi, G., Laursen, M. G., Dohn, A. O., Moltke, A., Vester, P., Hansen, F. K., Kjaer, K. S., Harlang, T., Hartsock, R., Christensen, M., Gaffney, K. J., Henriksen, N. E., Moller, K. B., Haldrup, K., Nielsen, M. M. 2018; 25: 306–15


    Time-resolved X-ray scattering patterns from photoexcited molecules in solution are in many cases anisotropic at the ultrafast time scales accessible at X-ray free-electron lasers (XFELs). This anisotropy arises from the interaction of a linearly polarized UV-Vis pump laser pulse with the sample, which induces anisotropic structural changes that can be captured by femtosecond X-ray pulses. In this work, a method for quantitative analysis of the anisotropic scattering signal arising from an ensemble of molecules is described, and it is demonstrated how its use can enhance the structural sensitivity of the time-resolved X-ray scattering experiment. This method is applied on time-resolved X-ray scattering patterns measured upon photoexcitation of a solvated di-platinum complex at an XFEL, and the key parameters involved are explored. It is shown that a combined analysis of the anisotropic and isotropic difference scattering signals in this experiment allows a more precise determination of the main photoinduced structural change in the solute, i.e. the change in Pt-Pt bond length, and yields more information on the excitation channels than the analysis of the isotropic scattering only. Finally, it is discussed how the anisotropic transient response of the solvent can enable the determination of key experimental parameters such as the instrument response function.

    View details for DOI 10.1107/S1600577517016964

    View details for Web of Science ID 000426315400002

    View details for PubMedID 29488907

  • Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2 '-bipyridine)(CN)(4)](2-) PHYSICAL CHEMISTRY CHEMICAL PHYSICS Kjaer, K. S., Kunnus, K., Harlang, T. B., Van Driel, T. B., Ledbetter, K., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Li, L., Laursen, M. G., Biasin, E., Hansen, F. B., Vester, P., Christensen, M., Haldrup, K., Nielsen, M. M., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Uhlig, J., Sundstom, V., Nemeth, Z., Szemes, D., Bajnoczi, E., Vanko, G., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Lemke, H. T., Canton, S. E., Warnmark, K., Persson, P., Cordones, A. A., Gaffney, K. J. 2018; 20 (6): 4238–49


    The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile.1,2 In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (5MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)4]2- allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.

    View details for PubMedID 29364300

  • L-edge spectroscopy of dilute, radiation-sensitive systems using a transition-edge-sensor array JOURNAL OF CHEMICAL PHYSICS Titus, C. J., Baker, M. L., Lee, S., Cho, H., Doriese, W. B., Fowler, J. W., Gaffney, K., Gard, J. D., Hilton, G. C., Kenney, C., Knight, J., Li, D., Marks, R., Minitti, M. P., Morgan, K. M., O'Neil, G. C., Reintsema, C. D., Schmidt, D. R., Sokaras, D., Swetz, D. S., Ullom, J. N., Weng, T., Williams, C., Young, B. A., Irwin, K. D., Solomon, E. I., Nordlund, D. 2017; 147 (21): 214201


    We present X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous ferricyanide. These measurements demonstrate the ability of high-throughput transition-edge-sensor (TES) spectrometers to access the rich soft X-ray (100-2000 eV) spectroscopy regime for dilute and radiation-sensitive samples. Our low-concentration data are in agreement with high-concentration measurements recorded by grating spectrometers. These results show that soft-X-ray RIXS spectroscopy acquired by high-throughput TES spectrometers can be used to study the local electronic structure of dilute metal-centered complexes relevant to biology, chemistry, and catalysis. In particular, TES spectrometers have a unique ability to characterize frozen solutions of radiation- and temperature-sensitive samples.

    View details for PubMedID 29221417

    View details for PubMedCentralID PMC5720893

  • Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2 '- bipyridine)(2)(CN)(2)] STRUCTURAL DYNAMICS Kjaer, K. S., Zhang, W., Alonso-Mori, R., Bergmann, U., Chollet, M., Hadt, R. G., Hartsock, R. W., Harlang, T., Kroll, T., Kubicek, K., Lemke, H. T., Liang, H. W., Liu, Y., Nielsen, M. M., Robinson, J. S., Solomon, E. I., Sokaras, D., van Driel, T. B., Weng, T., Zhu, D., Persson, P., Warnmark, K., Sundstrom, V., Gaffney, K. J. 2017; 4 (4): 044030


    We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2'-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2'-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2- in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6-2N]2N-4, where N = 1-3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.

    View details for PubMedID 28653021

  • Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy SCIENCE Mara, M. W., Hadt, R. G., Reinhard, M., Kroll, T., Lim, H., Hartsock, R. W., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Hodgson, K. O., Hedman, B., Bergmann, U., Gaffney, K. J., Solomon, E. I. 2017; 356 (6344): 1276-+


    The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.

    View details for PubMedID 28642436

    View details for PubMedCentralID PMC5706643

  • Probing ultrafast pi pi*/n pi* internal conversion in organic chromophores via K-edge resonant absorption NATURE COMMUNICATIONS Wolf, T. A., Myhre, R. H., Cryan, J. P., Coriani, S., Squibb, R. J., Battistoni, A., Berrah, N., Bostedt, C., Bucksbaum, P., Coslovich, G., Feifel, R., Gaffney, K. J., Grilj, J., Martinez, T. J., Miyabe, S., Moeller, S. P., Mucke, M., Natan, A., Obaid, R., Osipov, T., Plekan, O., Wang, S., Koch, H., Guehr, M. 2017; 8: 29


    Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method's impressive electronic structure sensitivity for excited-state investigations.Many photo-induced processes such as photosynthesis occur in organic molecules, but their femtosecond excited-state dynamics are difficult to track. Here, the authors exploit the element and site selectivity of soft X-ray absorption to sensitively follow the ultrafast ππ*/nπ* electronic relaxation of hetero-organic molecules.

    View details for PubMedID 28642477

  • Coherent structural trapping through wave packet dispersion during photoinduced spin state switching NATURE COMMUNICATIONS Lemke, H. T., Kjaer, K. S., Hartsock, R., van Driel, T. B., Chollet, M., Glownia, J. M., Song, S., Zhu, D., Pace, E., Matar, S. F., Nielsen, M. M., Benfatto, M., Gaffney, K. J., Collet, E., Cammarata, M. 2017; 8


    The description of ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be treated independently. Here we gain experimental insights, beyond the Born-Oppenheimer approximation, into the light-induced spin-state trapping dynamics of the prototypical [Fe(bpy)3](2+) compound by time-resolved X-ray absorption spectroscopy at sub-30-femtosecond resolution and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional molecules.

    View details for DOI 10.1038/ncomms15342

    View details for Web of Science ID 000401959300001

    View details for PubMedID 28537270

  • Charge and Spin-State Characterization of Cobalt Bis(o-dioxolene) Valence Tautomers Using Co K beta X-ray Emission and L-Edge X-ray Absorption Spectroscopies INORGANIC CHEMISTRY Liang, H. W., Kroll, T., Nordlund, D., Weng, T., Sokaras, D., Pierpont, C. G., Gaffney, K. J. 2017; 56 (2): 737-747


    The valence tautomeric states of Co(phen)(3,5-DBQ)2 and Co(tmeda)(3,5-DBQ)2, where 3,5-DBQ is either the semiquinone (SQ(-)) or catecholate (Cat(2-)) form of 3,5-di-tert-butyl-1,2-benzoquinone, have been examined by a series of cobalt-specific X-ray spectroscopies. In this work, we have utilized the sensitivity of 1s3p X-ray emission spectroscopy (Kβ XES) to the oxidation and spin states of 3d transition-metal ions to determine the cobalt-specific electronic structure of valence tautomers. A comparison of their Kβ XES spectra with the spectra of cobalt coordination complexes with known oxidation and spin states demonstrates that the low-temperature valence tautomer can be described as a low-spin Co(III) configuration and the high-temperature valence tautomer as a high-spin Co(II) configuration. This conclusion is further supported by Co L-edge X-ray absorption spectroscopy (L-edge XAS) of the high-temperature valence tautomers and ligand-field atomic-multiplet calculations of the Kβ XES and L-edge XAS spectra. The nature and strength of the magnetic exchange interaction between the cobalt center and SQ(-) in cobalt valence tautomers is discussed in view of the effective spin at the Co site from Kβ XES and the molecular spin moment from magnetic susceptibility measurements.

    View details for DOI 10.1021/acs.inorgchem.6b01666

    View details for Web of Science ID 000392262400011

    View details for PubMedID 28035824

  • Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution CHEMICAL SCIENCE Zhang, W., Kjaer, K. S., Alonso-Mori, R., Bergmann, U., Chollet, M., Fredin, L. A., Hadt, R. G., Hartsock, R. W., Harlang, T., Kroll, T., Kubicek, K., Lemke, H. T., Liang, H. W., Liu, Y., Nielsen, M. M., Persson, P., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Weng, T., Zhu, D., Warnmark, K., Sundstromb, V., Gaffney, K. J. 2017; 8 (1): 515-523


    Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)4(bpy)]2-. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)4(bpy)]2- decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2'-bipyridine)3]2+ by more than two orders of magnitude.

    View details for DOI 10.1039/c6sc03070j

    View details for Web of Science ID 000391454500060

    View details for PubMedCentralID PMC5341207

  • Anti-Stokes resonant x-ray Raman scattering for atom specific and excited state selective dynamics NEW JOURNAL OF PHYSICS Kunnus, K., Josefsson, I., Rajkovic, I., Schreck, S., Quevedo, W., Beye, M., Greubel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Gaffney, K. J., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., Techert, S., Wernet, P., Odelius, M., Foehlisch, A. 2016; 18
  • Viewing the Valence Electronic Structure of Ferric and Ferrous Hexacyanide in Solution from the Fe and Cyanide Perspectives JOURNAL OF PHYSICAL CHEMISTRY B Kunnus, K., Zhang, W., Delcey, M. G., Pinjari, R. V., Miedema, P. S., Schreck, S., Quevedo, W., Schroeder, H., Foehlisch, A., Gaffney, K. J., Lundberg, M., Odelius, M., Wernet, P. 2016; 120 (29): 7182-7194


    The valence-excited states of ferric and ferrous hexacyanide ions in aqueous solution were mapped by resonant inelastic X-ray scattering (RIXS) at the Fe L2,3 and N K edges. Probing of both the central Fe and the ligand N atoms enabled identification of the metal- and ligand-centered excited states, as well as ligand-to-metal and metal-to-ligand charge-transfer excited states. Ab initio calculations utilizing the RASPT2 method were used to simulate the Fe L2,3-edge RIXS spectra and enabled quantification of the covalencies of both occupied and empty orbitals of π and σ symmetry. We found that π back-donation in the ferric complex is smaller than that in the ferrous complex. This is evidenced by the relative amounts of Fe 3d character in the nominally 2π CN(-) molecular orbital of 7% and 9% in ferric and ferrous hexacyanide, respectively. Utilizing the direct sensitivity of Fe L3-edge RIXS to the Fe 3d character in the occupied molecular orbitals, we also found that the donation interactions are dominated by σ bonding. The latter was found to be stronger in the ferric complex, with an Fe 3d contribution to the nominally 5σ CN(-) molecular orbitals of 29% compared to 20% in the ferrous complex. These results are consistent with the notion that a higher charge at the central metal atom increases donation and decreases back-donation.

    View details for DOI 10.1021/acs.jpcb.6b04751

    View details for Web of Science ID 000380730300016

    View details for PubMedID 27380541

  • Identification of the dominant photochemical pathways and mechanistic insights to the ultrafast ligand exchange of Fe(CO)5 to Fe(CO)4EtOH. Structural dynamics Kunnus, K., JOSEFSSON, I., RAJKOVIC, I., Schreck, S., Quevedo, W., Beye, M., Weniger, C., Grübel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Gaffney, K. J., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., de Groot, F. M., Techert, S., Odelius, M., Wernet, P., Föhlisch, A. 2016; 3 (4): 043204-?


    We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)5 in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)4 which are observed following a charge transfer photoexcitation of Fe(CO)5 as reported in our previous study [Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the (1)A1 state of Fe(CO)4. A sub-picosecond time constant of the spin crossover from (1)B2 to (3)B2 is rationalized by the proposed (1)B2 → (1)A1 → (3)B2 mechanism. Ultrafast ligation of the (1)B2 Fe(CO)4 state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the (3)B2 Fe(CO)4 ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via (1)B2 → (1)A1 → (1)A' Fe(CO)4EtOH pathway and the time scale of the (1)A1 Fe(CO)4 state ligation is governed by the solute-solvent collision frequency. Our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution.

    View details for DOI 10.1063/1.4941602

    View details for PubMedID 26958587

    View details for PubMedCentralID PMC4752567

  • Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)(2)](2+) PHYSICAL REVIEW LETTERS Biasin, E., van Driel, T., Kjaer, K. S., Dohn, A. O., Christensen, M., Harlang, T., Chabera, P., Liu, Y., Uhlig, J., Papai, M., Nemeth, Z., Hartsock, R., Liang, W., Zhang, J., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Assefa, T. A., Britz, A., Galler, A., Gawelda, W., Bressler, C., Gaffney, K. J., Lemke, H. T., Moller, K. B., Nielsen, M. M., Sundstrom, V., Vanko, G., Warnmark, K., Canton, S. E., Haldrup, K. 2016; 117 (1): 013002


    We study the structural dynamics of photoexcited [Co(terpy)_{2}]^{2+} in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7  ps.

    View details for DOI 10.1103/PhysRevLett.117.013002

    View details for Web of Science ID 000378881300004

    View details for PubMedID 27419566

  • Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses NATURE COMMUNICATIONS Yang, J., Guehr, M., Vecchione, T., Robinson, M. S., Li, R., Hartmann, N., Shen, X., Coffee, R., Corbett, J., Fry, A., Gaffney, K., Gorkhover, T., Hast, C., Jobe, K., Makasyuk, I., Reid, A., Robinson, J., Vetter, S., Wang, F., Weathersby, S., Yoneda, C., Centurion, M., Wang, X. 2016; 7


    Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angström spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.

    View details for DOI 10.1038/ncomms11232

    View details for Web of Science ID 000373622400001

    View details for PubMedID 27046298

    View details for PubMedCentralID PMC4822053

  • Femtosecond gas phase electron diffraction with MeV electrons FARADAY DISCUSSIONS Yang, J., Guehr, M., Vecchione, T., Robinson, M. S., Li, R., Hartmann, N., Shen, X., Coffee, R., Corbett, J., Fry, A., Gaffney, K., Gorkhover, T., Hast, C., Jobe, K., Makasyuk, I., Reid, A., Robinson, J., Vetter, S., Wang, F., Weathersby, S., Yoneda, C., Wang, X., Centurion, M. 2016; 194: 563–81


    We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.

    View details for DOI 10.1039/c6fd00071a

    View details for Web of Science ID 000392422200026

    View details for PubMedID 27711826

  • Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)(5) in solution NATURE Wernet, P., Kunnus, K., JOSEFSSON, I., RAJKOVIC, I., Quevedo, W., Beye, M., Schreck, S., Gruebel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., de Groot, F. M., Gaffney, K. J., Techert, S., Odelius, M., Foehlisch, A. 2015; 520 (7545): 78-81


    Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site that need to be controlled to optimize complexes for photocatalytic hydrogen production and selective carbon-hydrogen bond activation. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst with an electron deficiency at the Fe centre, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16 - 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.

    View details for DOI 10.1038/nature14296

    View details for Web of Science ID 000352027700040

    View details for PubMedID 25832405

  • Mechanistic Studies of Photoinduced Spin Crossover and Electron Transfer in Inorganic Complexes ACCOUNTS OF CHEMICAL RESEARCH Zhang, W., Gaffney, K. J. 2015; 48 (4): 1140-1148


    Electronic excited-state phenomena provide a compelling intersection of fundamental and applied research interests in the chemical sciences. This holds true for coordination chemistry, where harnessing the strong optical absorption and photocatalytic activity of compounds depends on our ability to control fundamental physical and chemical phenomena associated with the nonadiabatic dynamics of electronic excited states. The central events of excited-state chemistry can critically influence the dynamics of electronic excited states, including internal conversion (transitions between distinct electronic states) and intersystem crossing (transitions between electronic states with different spin multiplicities), events governed by nonadiabatic interactions between electronic states in close proximity to conical intersections, as well as solvation and electron transfer. The diversity of electronic and nuclear dynamics also makes the robust interpretation of experimental measurements challenging. Developments in theory, simulation, and experiment can all help address the interpretation and understanding of chemical dynamics in organometallic and coordination chemistry. Synthesis presents the opportunity to chemically engineer the strength and symmetry of the metal-ligand interactions. This chemical control can be exploited to understand the influence of electronic ground state properties on electronic excited-state dynamics. New time-resolved experimental methods and the insightful exploitation of established methods have an important role in understanding, and ideally controlling, the photophysics and photochemistry of transition metal complexes. Techniques that can disentangle the coupled motion of electrons and nuclear dynamics warrant emphasis. We present a review of electron localization dynamics in charge transfer excited states and the dynamics of photoinitiated spin crossover dynamics. Both electron localization and spin crossover have been investigated by numerous research groups with femtosecond resolution spectroscopy, but challenges in experimental interpretation have left significant uncertainty about the molecular properties that control these phenomena. Our Account will emphasize how tailoring the experimental probe, femtosecond resolution vibrational anisotropy for electron localization, and femtosecond resolution hard X-ray fluorescence for spin crossover can make a significant impact on the interpretability of experimental measurements. The emphasis on thorough and robust interpretation has also led to an emphasis on simpler molecular systems. This enables iteration between experiment and theory, a requirement for the development of a more predictive understanding of electronic excited-state phenomena and an essential step to the development of design rules for solar materials.

    View details for DOI 10.1021/ar500407p

    View details for Web of Science ID 000353429400025

    View details for PubMedID 25789406

  • Ultrafast X-ray Auger probing of photoexcited molecular dynamics NATURE COMMUNICATIONS McFarland, B. K., Farrell, J. P., Miyabe, S., Tarantelli, F., Aguilar, A., Berrah, N., Bostedt, C., Bozek, J. D., Bucksbaum, P. H., Castagna, J. C., Coffee, R. N., Cryan, J. P., Fang, L., Feifel, R., Gaffney, K. J., Glownia, J. M., Martinez, T. J., Mucke, M., Murphy, B., Natan, A., Osipov, T., Petrovic, V. S., Schorb, S., Schultz, T., SPECTOR, L. S., Swiggers, M., Tenney, I., Wang, S., WHITE, J. L., White, W., Guehr, M. 2014; 5

    View details for DOI 10.1038/ncomms5235

    View details for Web of Science ID 000338840000004

  • Tracking excited-state charge and spin dynamics in iron coordination complexes. Nature Zhang, W., Alonso-Mori, R., Bergmann, U., Bressler, C., Chollet, M., Galler, A., Gawelda, W., Hadt, R. G., Hartsock, R. W., Kroll, T., Kjær, K. S., Kubicek, K., Lemke, H. T., Liang, H. W., Meyer, D. A., Nielsen, M. M., Purser, C., Robinson, J. S., Solomon, E. I., Sun, Z., Sokaras, D., van Driel, T. B., Vankó, G., Weng, T., Zhu, D., Gaffney, K. J. 2014; 509 (7500): 345-348


    Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

    View details for DOI 10.1038/nature13252

    View details for PubMedID 24805234

  • Contact Ion Pair Formation between Hard Acids and Soft Bases in Aqueous Solutions Observed with 2DIR Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY B Sun, Z., Zhang, W., Ji, M., Hartsock, R., Gaffney, K. J. 2013; 117 (49): 15306-15312


    The interaction of charged species in aqueous solution has important implications for chemical, biological, and environmental processes. We have used 2DIR spectroscopy to study the equilibrium dynamics of thiocyanate chemical exchange between free ion (NCS(-)) and contact ion pair configurations (MNCS(+)), where M(2+) = Mg(2+) or Ca(2+). Detailed studies of the influence of anion concentration and anion speciation show that the chemical exchange observed with the 2DIR measurements results from NCS(-) exchanging with other anion species in the first solvation shell surrounding Mg(2+) or Ca(2+). The presence of chemical exchange in the 2DIR spectra provides an indirect, but robust, determinant of contact ion pair formation. We observe preferential contact ion pair formation between soft Lewis base anions and hard Lewis acid cations. This observation cannot be easily reconciled with Pearson's acid-base concept or Collins' Law of Matching Water Affinities. The anions that form contact ion pairs also correspond to the ions with an affinity for water and protein surfaces, so similar physical and chemical properties may control these distinct phenomena.

    View details for DOI 10.1021/jp4033854

    View details for Web of Science ID 000328529000008

    View details for PubMedID 23895531

  • Fourier-transform inelastic X-ray scattering from time- and momentum-dependent phonon-phonon correlations NATURE PHYSICS Trigo, M., Fuchs, M., Chen, J., Jiang, M. P., Cammarata, M., Fahy, S., Fritz, D. M., Gaffney, K., Ghimire, S., Higginbotham, A., Johnson, S. L., Kozina, M. E., Larsson, J., Lemke, H., Lindenberg, A. M., Ndabashimiye, G., QUIRIN, F., Sokolowski-Tinten, K., Uher, C., Wang, G., Wark, J. S., Zhu, D., Reis, D. A. 2013; 9 (12): 790-794

    View details for DOI 10.1038/NPHYS2788

    View details for Web of Science ID 000327944600017

  • Aqueous Mg(2+) and Ca(2+) Ligand Exchange Mechanisms Identified with 2DIR Spectroscopy. journal of physical chemistry. B Sun, Z., Zhang, W., Ji, M., Hartsock, R., Gaffney, K. J. 2013; 117 (40): 12268-12275


    Biological systems must discriminate between calcium and magnesium for these ions to perform their distinct biological functions, but the mechanism for distinguishing aqueous ions has yet to be determined. Ionic recognition depends upon the rate and mechanism by which ligands enter and leave the first solvation shell surrounding these cations. We present a time-resolved vibrational spectroscopy study of these ligand exchange dynamics in aqueous solution. The sensitivity of the CN-stretch frequency of NCS(-) to ion pair formation has been utilized to investigate the mechanism and dynamics of ligand exchange into and out of the first solvation shell of aqueous magnesium and calcium ions with multidimensional vibrational (2DIR) spectroscopy. We have determined that anion exchange follows a dissociative mechanism for Mg(2+) and an associative mechanism for Ca(2+).

    View details for DOI 10.1021/jp407960x

    View details for PubMedID 24016251

  • Femtosecond X-ray Absorption Spectroscopy at a Hard X-ray Free Electron Laser: Application to Spin Crossover Dynamics JOURNAL OF PHYSICAL CHEMISTRY A Lemke, H. T., Bressler, C., Chen, L. X., Fritz, D. M., Gaffney, K. J., Galler, A., Gawelda, W., Haldrup, K., Hartsock, R. W., Ihee, H., Kim, J., Kim, K., Lee, J., Nielsen, M. M., Stickrath, A. B., Zhang, W., Zhu, D., Cammarata, M. 2013; 117 (4): 735–40


    X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (∼10(12) photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2'-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is ∼160 fs.

    View details for DOI 10.1021/jp312559h

    View details for Web of Science ID 000314492900005

    View details for PubMedID 23281652

  • A setup for resonant inelastic soft x-ray scattering on liquids at free electron laser light sources REVIEW OF SCIENTIFIC INSTRUMENTS Kunnus, K., Rajkovic, I., Schreck, S., Quevedo, W., Eckert, S., Beye, M., Suljoti, E., Weniger, C., Kalus, C., Gruebel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Gaffney, K. J., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., Techert, S., Wernet, P., Foehlisch, A. 2012; 83 (12)


    We present a flexible and compact experimental setup that combines an in vacuum liquid jet with an x-ray emission spectrometer to enable static and femtosecond time-resolved resonant inelastic soft x-ray scattering (RIXS) measurements from liquids at free electron laser (FEL) light sources. We demonstrate the feasibility of this type of experiments with the measurements performed at the Linac Coherent Light Source FEL facility. At the FEL we observed changes in the RIXS spectra at high peak fluences which currently sets a limit to maximum attainable count rate at FELs. The setup presented here opens up new possibilities to study the structure and dynamics in liquids.

    View details for DOI 10.1063/1.4772685

    View details for Web of Science ID 000312834300010

    View details for PubMedID 23277974

  • Site-Specific Measurement of Water Dynamics in the Substrate Pocket of Ketosteroid Isomerase Using Time-Resolved Vibrational Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY B Jha, S. K., Ji, M., Gaffney, K. J., Boxer, S. G. 2012; 116 (37): 11414-11421


    Little is known about the reorganization capacity of water molecules at the active sites of enzymes and how this couples to the catalytic reaction. Here, we study the dynamics of water molecules at the active site of a highly proficient enzyme, Δ(5)-3-ketosteroid isomerase (KSI), during a light-activated mimic of its catalytic cycle. Photoexcitation of a nitrile-containing photoacid, coumarin183 (C183), mimics the change in charge density that occurs at the active site of KSI during the first step of the catalytic reaction. The nitrile of C183 is exposed to water when bound to the KSI active site, and we used time-resolved vibrational spectroscopy as a site-specific probe to study the solvation dynamics of water molecules in the vicinity of the nitrile. We observed that water molecules at the active site of KSI are highly rigid, during the light-activated catalytic cycle, compared to the solvation dynamics observed in bulk water. On the basis of this result, we hypothesize that rigid water dipoles at the active site might help in the maintenance of the preorganized electrostatic environment required for efficient catalysis. The results also demonstrate the utility of nitrile probes in measuring the dynamics of local (H-bonded) water molecules in contrast to the commonly used fluorescence methods which measure the average behavior of primary and subsequent spheres of solvation.

    View details for DOI 10.1021/jp305225r1

    View details for Web of Science ID 000308855800003

    View details for PubMedID 22931297

    View details for PubMedCentralID PMC3461217

  • Resolving Photo-Induced Twisted Intramolecular Charge Transfer with Vibrational Anisotropy and TDDFT JOURNAL OF PHYSICAL CHEMISTRY B Zhang, W., Lan, Z., Sun, Z., Gaffney, K. J. 2012; 116 (37): 11527-11536


    The interplay between reaction environment and photochemical outcome has wide ranging implications for designing and directing light driven chemical conversions. We present a detailed mechanistic description of photoisomerization in julolidine malononitrile (JDMN) as the first step to characterizing this interplay between reaction pathways and reaction environment. We have used polarization resolved UV pump-mid-IR probe spectroscopy and time dependent DFT calculations to investigate the dynamics of charge transfer induced intramolecular rotation in JDMN. We have probed the mechanism and dynamics of photoisomerization with the symmetric and antisymmetric CN-stretch of the malononitrile group. These measurements show the S1 electronic excited state relaxes with a 12.3 ps time constant by isomerizing around both the C-C single and C-C double bond of the malononitrile group with a branching ratio of 1:5. Isomerization around the single bond leads to the formation of a metastable twisted excited state, while isomerization around the double bond leads to excited state quenching via a conical intersection between the S1 and S0 electronic states. We have characterized the electronic and nuclear structure of the long-lived excited state with pump-probe anisotropy measurements and time dependent DFT calculations using the CAM-B3LYP functional and the 6-31G(d,p) basis set. These calculations further confirm that isomerization around the malononitrile single bond forms a twisted intermolecular charge transfer excited state.

    View details for DOI 10.1021/jp306455m

    View details for Web of Science ID 000308855800015

    View details for PubMedID 22934677

  • Dynamics of Solvent-Mediated Electron Localization in Electronically Excited Hexacyanoferrate(III) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhang, W., Ji, M., Sun, Z., Gaffney, K. J. 2012; 134 (5): 2581-2588


    We have used polarization-resolved UV pump-mid-IR probe spectroscopy to investigate the dynamics of electron hole localization for excited-state ligand-to-metal charge-transfer (LMCT) excitation in Fe(CN)(6)(3-). The initially generated LMCT excited state has a single CN-stretch absorption band with no anisotropy. This provides strong evidence that this initial excited state preserves the octahedral symmetry of the electronic ground state by delocalizing the ligand hole in the LMCT excited state on all six cyanide ligands. This delocalized LMCT excited state decays to a second excited state with two CN-stretch absorption bands. We attribute both peaks to a single excited state because the formation time for both peaks matches the decay time for the delocalized LMCT excited state. The presence of two CN-stretch absorption bands demonstrates that this secondary excited state has lower symmetry. This observation, in conjunction with the solvent-dependent time constant for the formation of the secondary excited state, leads us to conclude that the secondary excited state corresponds to a LMCT state with a localized ligand hole.

    View details for DOI 10.1021/ja207306t

    View details for Web of Science ID 000300460600031

    View details for PubMedID 22233125

  • Influence of solute-solvent coordination on the orientational relaxation of ion assemblies in polar solvents JOURNAL OF CHEMICAL PHYSICS Ji, M., Hartsock, R. W., Sung, Z., Gaffney, K. J. 2012; 136 (1)


    We have investigated the rotational dynamics of lithium thiocyanate (LiNCS) dissolved in various polar solvents with time and polarization resolved vibrational spectroscopy. LiNCS forms multiple distinct ionic structures in solution that can be distinguished with the CN stretch vibrational frequency of the different ionic assemblies. By varying the solvent and the LiNCS concentration, the number and type of ionic structures present in solution can be controlled. Control of the ionic structure provides control over the volume, shape, and dipole moment of the solute, critical parameters for hydrodynamic and dielectric continuum models of friction. The use of solutes with sizes comparable to or smaller than the solvent molecules also helps amplify the sensitivity of the measurement to the short-ranged solute-solvent interaction. The measured orientational relaxation dynamics show many clear and distinct deviations from simple hydrodynamic behavior. All ionic structures in all solvents exhibit multi-exponential relaxation dynamics that do not scale with the solute volume. For Lewis base solvents such as benzonitrile, dimethyl carbonate, and ethyl acetate, the observed dynamics strongly show the effect of solute-solvent complex formation. For the weak Lewis base solvent nitromethane, we see no evidence for solute-solvent complex formation, but still see strong deviation from the predictions of simple hydrodynamic theory.

    View details for DOI 10.1063/1.3665140

    View details for Web of Science ID 000298967200021

    View details for PubMedID 22239783

  • Interdependence of Conformational and Chemical Reaction Dynamics during Ion Assembly in Polar Solvents JOURNAL OF PHYSICAL CHEMISTRY B Ji, M., Hartsock, R. W., Sun, Z., Gaffney, K. J. 2011; 115 (39): 11399-11408


    We have utilized time-resolved vibrational spectroscopy to study the interdependence of the conformational and chemical reaction dynamics of ion assembly in solution. We investigated the chemical interconversion dynamics of the LiNCS ion pair and the (LiNCS)(2) ion-pair dimer, as well as the spectral diffusion dynamics of these ionic assemblies. For the strongly coordinating Lewis base solvents benzonitrile, dimethyl carbonate, and ethyl acetate, we observe Li(+) coordination by both solvent molecules and NCS(-) anions, while the weak Lewis base solvent nitromethane shows no evidence for solvent coordination of Li(+) ions. The strong interaction between the ion-pair dimer structure and the Lewis base solvents leads to ion-pair dimer solvation dynamics that proceed more slowly than the ion-pair dimer dissociation. We have attributed the slow spectral diffusion dynamics to electrostatic reorganization of the solvent molecules coordinated to the Li(+) cations present in the ion-pair dimer structure and concluded that the dissociation of ion-pair dimers depends more critically on longer length scale electrostatic reorganization. This unusual inversion of the conformational and chemical reaction rates does not occur for ion-pair dimer dissociation in nitromethane or for ion pair association in any of the solvents.

    View details for DOI 10.1021/jp205660q

    View details for Web of Science ID 000295245400011

    View details for PubMedID 21854013

  • Direct measurement of the protein response to an electrostatic perturbation that mimics the catalytic cycle in ketosteroid isomerase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Jha, S. K., Ji, M., Gaffney, K. J., Boxer, S. G. 2011; 108 (40): 16612-16617


    Understanding how electric fields and their fluctuations in the active site of enzymes affect efficient catalysis represents a critical objective of biochemical research. We have directly measured the dynamics of the electric field in the active site of a highly proficient enzyme, Δ(5)-3-ketosteroid isomerase (KSI), in response to a sudden electrostatic perturbation that simulates the charge displacement that occurs along the KSI catalytic reaction coordinate. Photoexcitation of a fluorescent analog (coumarin 183) of the reaction intermediate mimics the change in charge distribution that occurs between the reactant and intermediate state in the steroid substrate of KSI. We measured the electrostatic response and angular dynamics of four probe dipoles in the enzyme active site by monitoring the time-resolved changes in the vibrational absorbance (IR) spectrum of a spectator thiocyanate moiety (a quantitative sensor of changes in electric field) placed at four different locations in and around the active site, using polarization-dependent transient vibrational Stark spectroscopy. The four different dipoles in the active site remain immobile and do not align to the changes in the substrate electric field. These results indicate that the active site of KSI is preorganized with respect to functionally relevant changes in electric fields.

    View details for DOI 10.1073/pnas.1113874108

    View details for Web of Science ID 000295536000031

    View details for PubMedID 21949360

    View details for PubMedCentralID PMC3189056

  • Characterizing the Deformational Isomers of Bimetallic Ir-2(dimen)(4)(2+) (dimen=1,8-diisocyano-p-menthane) with Vibrational Wavepacket Dynamics JOURNAL OF PHYSICAL CHEMISTRY A Hartsock, R. W., Zhang, W., Hill, M. G., Sabat, B., Gaffney, K. J. 2011; 115 (14): 2920-2926


    We studied the Ir(2)(dimen)(4)(2+) complex with ultrafast transient absorption spectroscopy and density functional theory and concluded that it possesses two singlet ground state isomers in room temperature solution. The molecule can adopt either a paddle wheel or a propeller conformation in solution, where the paddle wheel structure possesses a metal-metal bond of 4.4 Å and a dihedral angle between the quasi-C(4v) planes of 0° and the propeller structure has a metal-metal bond of 3.6 Å and a dihedral angle of 17° when crystallized. Each conformation has a distinct absorption in the visible attributed to a (1)(dσ(z)* → pσ(z)) excitation, with the long eclipsed structure absorbing at 475 nm and the short twisted structure absorbing at 585 nm. We independently pumped at each of these visible transitions to form vibrational wavepackets on the ground and excited state potential energy surfaces, which modulated the ground state bleach and stimulated emission signals, respectively. We found that the ground state wavepacket oscillates with a frequency of 48 cm(-1) when pumping the red peak and 11 cm(-1) when pumping the blue peak. We assign these frequencies to the Ir-Ir symmetric stretch, with the variation in frequency reflecting the variation in metal-metal bond strength in support of our assignment of the blue peak to the longer Ir-Ir bond length conformer and the red peak to the shorter Ir-Ir bond length conformer. When pumping the red peak, we found two modes with frequencies of 80 and 119 cm(-1) in the stimulated emission and only one mode at 75 cm(-1) when pumping the blue peak. We assign the 75-80 cm(-1) frequency to the Ir-Ir stretch and the 119 cm(-1) vibration to the dihedral angle twist in the excited state. The variation in the excited state dynamics does not result from the excitation of different electronic states, but rather from excitation to different Franck-Condon regions of the same electronic excited state potential energy surface. This occurs because of the large difference in ground state molecular structure. DFT calculations support the existence of a single electronic excited state being accessed from two distinct structural isomers with conformations similar to those observed with X-ray crystallography.

    View details for DOI 10.1021/jp1114493

    View details for Web of Science ID 000289215500002

    View details for PubMedID 21428426

  • H-bond switching and ligand exchange dynamics in aqueous ionic solution CHEMICAL PHYSICS LETTERS Gaffney, K. J., Ji, M., Odelius, M., Park, S., Sun, Z. 2011; 504 (1-3): 1-6
  • Orientational relaxation dynamics in aqueous ionic solution: Polarization-selective two-dimensional infrared study of angular jump-exchange dynamics in aqueous 6M NaClO4 JOURNAL OF CHEMICAL PHYSICS Ji, M., Gaffney, K. J. 2011; 134 (4)


    The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynamics of water rotation. We have used polarization resolved ultrafast two-dimensional infrared (2DIR) spectroscopy to investigate the rotational dynamics of deuterated hydroxyl groups (OD) in a solution of 6M NaClO(4) dissolved in isotopically mixed water. Aqueous 6M NaClO(4) has two peaks in the OD stretching region, one associated with hydroxyl groups that donate a H-bond to another water molecule (OD(W)) and one associated with hydroxyl groups that donate a H-bond to a perchlorate anion (OD(P)). Two-dimensional IR spectroscopy temporally resolves the equilibrium inter conversion of these spectrally distinct H-bond configurations, while polarization-selective 2DIR allows us to access the orientational motions associated with this chemical exchange. We have developed a general jump-exchange kinetic theory to model angular jumps associated with chemical exchange events. We use this to model polarization-selective 2DIR spectra and pump-probe anisotropy measurements. We determine the H-bond exchange induced jump angle to be 49 ± 5° and the H-bond exchange rate to be 6 ± 1 ps. Additionally, the separation of the 2DIR signal into contributions that have or have not undergone H-bond exchange allows us to directly determine the orientational dynamics of the OD(W) and the OD(P) configurations without contributions from the exchanged population. This proves to be important because the orientational relaxation dynamics of the populations that have undergone a H-bond exchange differ significantly from the populations that remain in one H-bond configuration. We have determined the slow orientational relaxation time constant to be 6.0 ± 1 ps for the OD(W) configuration and 8.3 ± 1 ps for the OD(P) configuration. We conclude from these measurements that the orientational dynamics of hydroxyl groups in distinct H-bond configurations do differ, but not significantly.

    View details for DOI 10.1063/1.3530783

    View details for Web of Science ID 000286897600082

    View details for PubMedID 21280757

  • Dynamics of Ion Assembly in Solution: 2DIR Spectroscopy Study of LiNCS in Benzonitrile JOURNAL OF PHYSICAL CHEMISTRY LETTERS Ji, M., Park, S., Gaffney, K. J. 2010; 1 (12): 1771-1775
  • Ligand Exchange Dynamics in Aqueous Solution Studied with 2DIR Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY B Park, S., Ji, M., Gaffney, K. J. 2010; 114 (19): 6693-6702


    We have used time-resolved multidimensional vibrational spectroscopy, generally termed 2DIR spectroscopy, to study the equilibrium dynamics of ligand exchange in an aqueous solution containing 3.4 M Mg(ClO(4))(2) and 1.2 M NaSCN. The sensitivity of the CN stretching frequency of thiocyanate (SCN(-)) to contact ion pair formation with Mg(2+) ions generates distinct spectroscopic signatures for the MgNCS(+) contact ion pair and the free SCN(-). We have utilized 2DIR spectroscopy to successfully resolve the interconversion between these thiocyanate configurations and measured the MgNCS(+) contact ion pair dissociation time constant to be 52 +/- 10 ps. We attribute the observed dynamics to perchlorate-thiocyanate anion exchange in the first solvation shell of the Mg(2+) cation. Magnesium ions in this concentrated ionic solution will be coordinated by water molecules, as well as perchlorate and thiocyanate ions. While prior studies have observed microsecond residence times for water ligands in the first coordination sphere of Mg(2+), our study represents the first experimental observation of anion exchange in the first solvent shell of the Mg(2+) cation. We have also used orientational relaxation and spectral diffusion dynamics to quantify the dynamical distinctions between the free anion and the anion in the contact ion pair.

    View details for DOI 10.1021/jp100833t

    View details for Web of Science ID 000277499700053

    View details for PubMedID 20426448

  • Characterization of charge transfer excitations in hexacyanomanganate(III) with Mn K-edge resonant inelastic x-ray scattering JOURNAL OF CHEMICAL PHYSICS Meyer, D. A., Zhang, X., Bergmann, U., Gaffney, K. J. 2010; 132 (13)


    We use hard x-ray resonant inelastic x-ray scattering (RIXS) and density functional theory (DFT) calculations to characterize charge transfer excitations in K(3)Mn(CN)(6). The combination of RIXS measurements and DFT calculations allows us to characterize the strength of the ligand-metal electronic interaction and assign the Raman resonances in the RIXS spectra to charge transfer excitations. With x-ray excitation energies resonant with the T(2g) and E(g) pre-edge peaks derived predominantly from the Mn 3d orbitals, we observe Raman resonances in the energy transfer range from 2 to 12 eV, which results from the filling of the 1s core-hole from T(1u)-symmetry occupied orbitals. DFT calculations indicate that these orbitals exhibit primarily ligand character, supporting the assignment of the energy transfer resonances to ligand-to-metal charge transfer excitations. Our RIXS measurements and DFT calculations also indicate that the E(g)-orbital spin-splits by roughly 0.8 eV, though we do not cleanly resolve the two absorption peaks in the RIXS spectra. We also see evidence for a metal-to-ligand charge transfer (MLCT) excitation when exciting with a 6545.0 eV incident photon, roughly 4 eV above the E(g) absorption peaks. The 6545.0 eV resonant emission spectrum shows a 6.0 eV energy transfer resonance, which corresponds to a final state hole in the T(2g) partially occupied orbital. DFT calculations indicate that excitation at 6545.0 eV populates an unoccupied T(1u)-symmetry orbital of primarily ligand character. Given the predominantly metal character of the final state hole, we assign the 6.0 eV Raman resonance to a MLCT excitation. These measurements demonstrate the ability of hard x-ray RIXS to characterize the valence electronic structure of coordination compounds.

    View details for DOI 10.1063/1.3367958

    View details for Web of Science ID 000276972600024

    View details for PubMedID 20387936

  • Atomic resolution mapping of the excited-state electronic structure of Cu2O with time-resolved x-ray absorption spectroscopy PHYSICAL REVIEW B Hillyard, P. W., Kuchibhatla, S. V., Glover, T. E., Hertlein, M. P., Huse, N., Nachimuthu, P., Saraf, L. V., Thevuthasan, S., Gaffney, K. J. 2009; 80 (12)
  • Ultrafast Dynamics of Hydrogen Bond Exchange in Aqueous Ionic Solutions JOURNAL OF PHYSICAL CHEMISTRY B Park, S., Odelius, M., Gaffney, K. J. 2009; 113 (22): 7825-7835


    The structural and dynamical properties of aqueous ionic solutions influence a wide range of natural and biological processes. In these solutions, water has the opportunity to form hydrogen bonds with other water molecules and anions. Knowing the time scale with which these configurations interconvert represents a key factor to understanding the influence of molecular scale heterogeneity on chemical events in aqueous ionic solutions. We have used ultrafast IR spectroscopy and Car-Parrinello molecular dynamics (CPMD) simulations to investigate the hydrogen bond (H-bond) structural dynamics in aqueous 6 M sodium perchlorate (NaClO4) solution. We have measured the H-bond exchange dynamics between spectrally distinct water-water and water-anion H-bond configurations with 2DIR spectroscopy and the orientational relaxation dynamics of water molecules in different H-bond configurations with polarization-selective IR pump-probe experiments. The experimental H-bond exchange time correlates strongly with the experimental orientational relaxation time of water molecules. This agrees with prior observations in water and aqueous halide solutions, and has been interpreted within the context of an orientational jump model for the H-bond exchange. The CPMD simulations performed on aqueous 6 M NaClO4 solution clearly demonstrate that water molecules organize into two radially and angularly distinct structural subshells within the first solvation shell of the perchlorate anion, with one subshell possessing the majority of the water molecules that donate H-bonds to perchlorate anions and the other subshell possessing predominantly water molecules that donate two H-bonds to other water molecules. Due to the high ionic concentration used in the simulations, essentially all water molecules reside in the first ionic solvation shells. The CPMD simulations also demonstrate that the molecular exchange between these two structurally distinct subshells proceeds more slowly than the H-bond exchange between the two spectrally distinct H-bond configurations. We interpret this to indicate that orientational motions predominantly dictate the rate of H-bond exchange, while translational diffusion must occur to complete the molecular exchange between the two structurally distinct subshells around the perchlorate anions. The 2DIR measurements observe the H-bond exchange between the two spectrally distinct H-bond configurations, but the lifetime of the hydroxyl stretch precludes the observation of the slower molecular exchange. Our 2DIR experiments and CPMD simulations demonstrate that orientational motions predominantly equilibrate water molecules within their local solvation subshells, but the full molecular equilibration within the first solvation shell around the perchlorate anion necessitates translational motion.

    View details for DOI 10.1021/jp9016739

    View details for Web of Science ID 000266545500014

    View details for PubMedID 19435307

  • Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation NANO LETTERS Ji, M., Park, S., Connor, S. T., Mokari, T., Cui, Y., Gaffney, K. J. 2009; 9 (3): 1217-1222


    We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.

    View details for DOI 10.1021/nl900103f

    View details for Web of Science ID 000264142100052

    View details for PubMedID 19226125

  • Carrier-induced disordering dynamics in InSb studied with density functional perturbation theory PHYSICAL REVIEW B Hillyard, P. B., Reis, D. A., Gaffney, K. J. 2008; 77 (19)
  • X-ray diffuse scattering measurements of nucleation dynamics at femtosecond resolution PHYSICAL REVIEW LETTERS Lindenberg, A. M., Engemann, S., Gaffney, K. J., Sokolowski-Tinten, K., Larsson, J., Hillyard, P. B., Reis, D. A., Fritz, D. M., ARTHUR, J., Akre, R. A., George, M. J., Deb, A., Bucksbaum, P. H., Hajdu, J., Meyer, D. A., Nicoul, M., Blome, C., Tschentscher, T., Cavalieri, A. L., Falcone, R. W., Lee, S. H., Pahl, R., Rudati, J., Fuoss, P. H., Nelson, A. J., Krejcik, P., Siddons, D. P., Lorazo, P., Hastings, J. B. 2008; 100 (13)


    Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process.

    View details for DOI 10.1103/PhysRevLett.100.135502

    View details for Web of Science ID 000254670300046

    View details for PubMedID 18517965

  • Imaging atomic structure and dynamics with ultrafast X-ray scattering SCIENCE Gaffney, K. J., Chapman, H. N. 2007; 316 (5830): 1444-1448


    Measuring atomic-resolution images of materials with x-ray photons during chemical reactions or physical transformations resides at the technological forefront of x-ray science. New x-ray-based experimental capabilities have been closely linked with advances in x-ray sources, a trend that will continue with the impending arrival of x-ray-free electron lasers driven by electron accelerators. We discuss recent advances in ultrafast x-ray science and coherent imaging made possible by linear-accelerator-based light sources. These studies highlight the promise of ultrafast x-ray lasers, as well as the technical challenges and potential range of applications that will accompany these transformative x-ray light sources.

    View details for Web of Science ID 000247066400033

    View details for PubMedID 17556577

  • Carrier-density-dependent lattice stability in InSb PHYSICAL REVIEW LETTERS Hillyard, P. B., Gaffney, K. J., Lindenberg, A. M., Engemann, S., Akre, R. A., ARTHUR, J., Blome, C., Bucksbaum, P. H., Cavalieri, A. L., Deb, A., Falcone, R. W., Fritz, D. M., Fuoss, P. H., Hajdu, J., Krejcik, P., Larsson, J., Lee, S. H., Meyer, D. A., Nelson, A. J., Pahl, R., Reis, D. A., Rudati, J., Siddons, D. P., Sokolowski-Tinten, K., von der Linde, D., Hastings, J. B. 2007; 98 (12)


    The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering.

    View details for DOI 10.1103/PhysRevLett.98.125501

    View details for PubMedID 17501133

  • Ultrafast bond softening in bismuth: Mapping a solid's interatomic potential with X-rays SCIENCE Fritz, D. M., Reis, D. A., Adams, B., Akre, R. A., ARTHUR, J., Blome, C., Bucksbaum, P. H., Cavalieri, A. L., Engemann, S., Fahy, S., Falcone, R. W., Fuoss, P. H., Gaffney, K. J., George, M. J., Hajdu, J., Hertlein, M. P., Hillyard, P. B., Hoegen, M. H., Kammler, M., Kaspar, J., Kienberger, R., Krejcik, P., Lee, S. H., Lindenberg, A. M., McFarland, B., Meyer, D., Montagne, T., Murray, E. D., Nelson, A. J., Nicoul, M., Pahl, R., Rudati, J., Schlarb, H., Siddons, D. P., Sokolowski-Tinten, K., Tschentscher, T., von der Linde, D., Hastings, J. B. 2007; 315 (5812): 633-636


    Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

    View details for DOI 10.1126/science.1135009

    View details for Web of Science ID 000243909400039

    View details for PubMedID 17272718

  • Ultrafast dynamics of laser-excited solids MRS BULLETIN Reis, D. A., Gaffney, K. J., Gilmer, G. H., Torralva, B. 2006; 31 (8): 601-606
  • Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb PHYSICAL REVIEW LETTERS Gaffney, K. J., Lindenberg, A. M., Larsson, J., Sokolowski-Tinten, K., Blome, C., Synnergren, O., Sheppard, J., Caleman, C., MacPhee, A. G., Weinstein, D., Lowney, D. P., Allison, T., Matthews, T., Falcone, R. W., Cavalieri, A. L., Fritz, D. M., Lee, S. H., Bucksbaum, P. H., Reis, D. A., Rudati, J., Macrander, A. T., Fuoss, P. H., Kao, C. C., Siddons, D. P., Pahl, R., Moffat, K., Als-Nielsen, J., Duesterer, S., Ischebeck, R., Schlarb, H., Schulte-Schrepping, H., Schneider, J., von der Linde, D., Hignette, O., Sette, F., Chapman, H. N., Lee, R. W., Hansen, T. N., Wark, J. S., Bergh, M., Huldt, G., van der Spoel, D., Timneanu, N., Hajdu, J., Akre, R. A., Bong, E., Krejcik, P., ARTHUR, J., Brennan, S., Luening, K., Hastings, J. B. 2005; 95 (12)


    The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

    View details for DOI 10.1103/PhysRevLett.95.125701

    View details for Web of Science ID 000231908200033

    View details for PubMedID 16197085

  • Atomic-scale visualization of inertial dynamics SCIENCE Lindenberg, A. M., Larsson, J., Sokolowski-Tinten, K., Gaffney, K. J., Blome, C., Synnergren, O., Sheppard, J., Caleman, C., MacPhee, A. G., Weinstein, D., Lowney, D. P., Allison, T. K., Matthews, T., Falcone, R. W., Cavalieri, A. L., Fritz, D. M., Lee, S. H., Bucksbaum, P. H., Reis, D. A., Rudati, J., Fuoss, P. H., Kao, C. C., Siddons, D. P., Pahl, R., Als-Nielsen, J., Duesterer, S., Ischebeck, R., Schlarb, H., Schulte-Schrepping, H., Tschentscher, T., Schneider, J., von der Linde, D., Hignette, O., Sette, F., Chapman, H. N., Lee, R. W., Hansen, T. N., Techert, S., Wark, J. S., Bergh, M., Huldt, G., van der Spoel, D., Timneanu, N., Hajdu, J., Akre, R. A., Bong, E., Krejcik, P., ARTHUR, J., Brennan, S., Luening, K., Hastings, J. B. 2005; 308 (5720): 392-395


    The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

    View details for DOI 10.1126/science.1107996

    View details for Web of Science ID 000228492000046

    View details for PubMedID 15831753

  • Clocking femtosecond x rays PHYSICAL REVIEW LETTERS Cavalieri, A. L., Fritz, D. M., Lee, S. H., Bucksbaum, P. H., Reis, D. A., Rudati, J., Mills, D. M., Fuoss, P. H., Stephenson, G. B., Kao, C. C., Siddons, D. P., Lowney, D. P., MacPhee, A. G., Weinstein, D., Falcone, R. W., Pahl, R., Als-Nielsen, J., Blome, C., Dusterer, S., Ischebeck, R., Schlarb, H., Schulte-Schrepping, H., Tschentscher, T., Schneider, J., Hignette, O., Sette, F., Sokolowski-Tinten, K., Chapman, H. N., Lee, R. W., Hansen, T. N., Synnergren, O., Larsson, J., Techert, S., Sheppard, J., Wark, J. S., Bergh, M., Caleman, C., Huldt, G., van der Spoel, D., Timneanu, N., Hajdu, J., Akre, R. A., Bong, E., Emma, P., Krejcik, P., ARTHUR, J., Brennan, S., Gaffney, K. J., Lindenberg, A. M., Luening, K., Hastings, J. B. 2005; 94 (11)


    Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

    View details for DOI 10.1103/PhysRevLett.94.114801

    View details for Web of Science ID 000227923200034

    View details for PubMedID 15903864

  • Measurement and dynamics of the spatial distribution of an electron localized at a metal-dielectric interface JOURNAL OF CHEMICAL PHYSICS Bezel, Gaffney, K. J., Garrett-Roe, S., Liu, S. H., Miller, A. D., Szymanski, P., Harris, C. B. 2004; 120 (2): 845–56


    The ability of time- and angle-resolved two-photon photoemission to estimate the size distribution of electron localization in the plane of a metal-adsorbate interface is discussed. It is shown that the width of angular distribution of the photoelectric current is inversely proportional to the electron localization size within the most common approximations in the description of image potential states. The localization of the n=1 image potential state for two monolayers of butyronitrile on Ag(111) is used as an example. For the delocalized n=1 state, the shape of the signal amplitude as a function of momentum parallel to the surface changes rapidly with time, indicating efficient intraband relaxation on a 100 fs time scale. For the localized state, little change was observed. The latter is related to the constant size distribution of electron localization, which is estimated to be a Gaussian with a 15+/-4 A full width at half maximum in the plane of the interface. A simple model was used to study the effect of a weak localization potential on the overall width of the angular distribution of the photoemitted electrons, which exhibited little sensitivity to the details of the potential. This substantiates the validity of the localization size estimate.

    View details for DOI 10.1063/1.1632386

    View details for Web of Science ID 000187718200037

    View details for PubMedID 15267921

  • Hydrogen bond breaking probed with multidimensional stimulated vibrational echo correlation spectroscopy JOURNAL OF CHEMICAL PHYSICS Asbury, J. B., Steinel, T., Stromberg, C., Gaffney, K. J., Piletic, I. R., Fayer, M. D. 2003; 119 (24): 12981-12997

    View details for DOI 10.1063/1.1627762

    View details for Web of Science ID 000187576300033

  • Hydrogen bond dynamics probed with ultrafast infrared heterodyne-detected multidimensional vibrational stimulated echoes PHYSICAL REVIEW LETTERS Asbury, J. B., Steinel, T., Stromberg, C., Gaffney, K. J., Piletic, I. R., Goun, A., Fayer, M. D. 2003; 91 (23)


    Hydrogen bond dynamics are explicated with exceptional detail using multidimensional infrared vibrational echo correlation spectroscopy with full phase information. Probing the hydroxyl stretch of methanol-OD oligomers in CCl4, the dynamics of the evolving hydrogen bonded network are measured with ultrashort (<50 fs) pulses. The data along with detailed model calculations demonstrate that vibrational relaxation leads to selective hydrogen bond breaking on the red side of the spectrum (strongest hydrogen bonds) and the production of singly hydrogen bonded photoproducts.

    View details for DOI 10.1103/PhysRevLett.91.237402

    View details for Web of Science ID 000187004500053

    View details for PubMedID 14683215

  • Structural dynamics of hydrogen bonded methanol oligomers: Vibrational transient hole burning studies of spectral diffusion JOURNAL OF CHEMICAL PHYSICS Piletic, I. R., Gaffney, K. J., Fayer, M. D. 2003; 119 (1): 423-434

    View details for DOI 10.1063/1.1578058

    View details for Web of Science ID 000183585400049

  • Ultrafast heterodyne detected infrared multidimensional vibrational stimulated echo studies of hydrogen bond dynamics CHEMICAL PHYSICS LETTERS Asbury, J. B., Steinel, T., Stromberg, C., Gaffney, K. J., Piletic, I. R., Goun, A., Fayer, M. D. 2003; 374 (3-4): 362-371
  • Orientational relaxation and vibrational excitation transfer in methanol-carbon tetrachloride solutions JOURNAL OF CHEMICAL PHYSICS Gaffney, K. J., Piletic, I. R., Fayer, M. D. 2003; 118 (5): 2270-2278

    View details for DOI 10.1063/1.1534580

    View details for Web of Science ID 000180579900030

  • Hydrogen bond dissociation and reformation in methanol oligomers following hydroxyl stretch relaxation JOURNAL OF PHYSICAL CHEMISTRY A Gaffney, K. J., Davis, P. H., Piletic, I. R., Levinger, N. E., Fayer, M. D. 2002; 106 (50): 12012-12023

    View details for DOI 10.1021/jp.021696g

    View details for Web of Science ID 000179921000004

  • Direct observation of two-dimensional electron solvation at alcohol/Ag(111) interfaces JOURNAL OF PHYSICAL CHEMISTRY B Liu, S. H., Miller, A. D., Gaffney, K. J., Szymanski, P., Garrett-Roe, S., Bezel, Harris, C. B. 2002; 106 (50): 12908–15

    View details for DOI 10.1021/jp025772r

    View details for Web of Science ID 000179921100010

  • Hydrogen bond breaking and reformation in alcohol oligomers following vibrational relaxation of a non-hydrogen-bond donating hydroxyl stretch JOURNAL OF PHYSICAL CHEMISTRY A Gaffney, K. J., Piletic, I. R., Fayer, M. D. 2002; 106 (41): 9428-9435

    View details for DOI 10.1021/jp021170w

    View details for Web of Science ID 000178548500007

  • Evolution of a two-dimensional band structure at a self-assembling interface JOURNAL OF PHYSICAL CHEMISTRY A Miller, A. D., Gaffney, K. J., Liu, S. H., Szymanski, P., Garrett-Roe, S., Wong, C. M., Harris, C. B. 2002; 106 (33): 7636–38

    View details for DOI 10.1021/jp025555q

    View details for Web of Science ID 000177472700021

  • Electron solvation in two dimensions SCIENCE Miller, A. D., Bezel, Gaffney, K. J., Garrett-Roe, S., Liu, S. H., Szymanski, P., Harris, C. B. 2002; 297 (5584): 1163–66


    Ultrafast two-photon photoemission has been used to study electron solvation at two-dimensional metal/polar-adsorbate interfaces. The molecular motion that causes the excess electron solvation is manifested as a dynamic shift in the electronic energy. Although the initially excited electron is delocalized in the plane of the interface, interactions with the adsorbate can lead to its localization. A method for determining the spatial extent of the localized electron in the plane of the interface has been developed. This spatial extent was measured to be on the order of a single adsorbate molecule.

    View details for DOI 10.1126/science.1073571

    View details for Web of Science ID 000177447600044

    View details for PubMedID 12183625

  • Femtosecond dynamics of electrons photoinjected into organic semiconductors at aromatic-metal interfaces JOURNAL OF PHYSICAL CHEMISTRY B Gaffney, K. J., Miller, A. D., Liu, S. H., Harris, C. B. 2001; 105 (38): 9031–39

    View details for DOI 10.1021/jp010931c

    View details for Web of Science ID 000171214200003

  • The adsorbate electron affinity dependence of femtosecond electron dynamics at dielectric/metal interfaces Gaffney, K. J., Liu, S. H., Miller, A. D., Szymanski, P., Harris, C. B. WILEY-V C H VERLAG GMBH. 2000: 759–63
  • Femtosecond electron dynamics at the benzene/Ag(111) interface CHEMICAL PHYSICS Gaffney, K. J., Wong, C. M., Liu, S. H., Miller, A. D., McNeill, J. D., Harris, C. B. 2000; 251 (1-3): 99–110
  • Femtosecond studies of electron dynamics at dielectric-metal interfaces JOURNAL OF PHYSICAL CHEMISTRY B Wong, C. M., McNeill, J. D., Gaffney, K. J., Ge, N. H., Miller, A. D., Liu, S. H., Harris, C. B. 1999; 103 (2): 282–92

    View details for DOI 10.1021/jp983913c

    View details for Web of Science ID 000079042900002

  • Femtosecond dynamics of electron localization at interfaces SCIENCE Ge, N. H., Wong, C. M., Lingle, R. L., McNeill, J. D., Gaffney, K. J., Harris, C. B. 1998; 279 (5348): 202–5